1. Field of the Invention
The present invention pertains to cellular telecommunications, and particularly to synchronization of timing units located in a cellular network such as a code division multiple access (CDMA) cellular network, for example.
2. Related Art and Other Considerations
In mobile telecommunications, a mobile station such as mobile telephone communicates over radio channels with base stations. Typically a plurality of base stations are, in turn, ultimately connected by an upper node, such as a radio network controller (RNC), to a mobile switching center (SC). The mobile switching center (MSC) is usually connected, e.g., via a gateway, to other telecommunication networks, such as the public switched telephone network.
In a code division multiple access (CDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a mathematical code (such as channelizing and scrambling codes) to distinguish it from information for other mobile stations which are utilizing the same radio frequency. Thus, in CDMA, the individual radio links are discriminated on the basis of codes. Various aspects of CDMA are set forth in Garg, Vijay K. et al., Applications of CDMA in Wireless/Personal Communications, Prentice Hall (1997).
In addition, in CDMA mobile communications, on the downlink typically the same baseband signal with suitable codes is sent from several base stations with overlapping coverage. In other words, frames with equal user data are sent from different base stations simultaneously on the downlink to the mobile station. The mobile terminal can thus receive and use signals from several base stations simultaneously. Moreover, since the radio environment changes rapidly, a mobile station likely has radio channels to several base stations at the same moment, e.g., so that the mobile station can select or combine the best channel and, if necessary, use signals directed to the mobile from various base stations in order to keep radio interference low and capacity high.
On the uplink, user data sent in frames from the mobile station for the mobile connection is received in multiple base stations. The mobile station transmits with the lowest power which is requested by one of the base stations. The base station that requires the lowest power can best xe2x80x9chearxe2x80x9d the mobile station. However, while interference is kept low, which base station has the best quality reception of the signal of the mobile station may randomly change during the course of the connection. Therefore, an upper node such as radio network controller (RNC) selects best quality ones of comparably number frames received from the mobile station by the base stations.
This utilization of radio channels between multiple base stations and a mobile station in a CDMA scheme, as summarized above, is termed xe2x80x9csoft handover.xe2x80x9d
The principles of diversity and soft-handover require that the base stations participating in a particular connection be synchronized relative to the upper node, e.g., to the radio network controller (RNC). Synchronization is required since, among other things, the plural base stations participating in a connection must send the same frame information at the same time to the mobile station involved in the connection.
Various techniques for obtaining synchronization between base stations and the upper node are described in U.S. Pat. No. 5,388,102 to Griffith et al. Such techniques include earth-orbiting satellite (e.g., utilizing the Global Positioning System [GPS]), a dedicated synchronization link, and interrupting synchronization signals on data links. GPS is used to obtain an absolute phase difference of (for example) 3 to 10 micro seconds (as occurs, for example, in IS-95). In other systems, it is sufficient if the base stations are synchronized with other phase differences (e.g., 2, 5, or 10 milliseconds) if the soft-handover procedure is assisted by the mobile station advising of the magnitude of the phase difference from a source to target base station (e.g., a mobile assisted handover, MAHO).
In U.S. Pat. No. 5,245,634 to Averbuch, loss of GPS causes a base station to send a synchronization message to a central site. The central site starts a measurement counter, and sends a master sequence to the base station. The base station then sends a return sequence. After M loops of such sequence with respective counting at both sites, the central site sends its measured round-trip time delay to the base station. The base station performs calculations (e.g., drift) and compensates for discrepancies in the round-trip time delay measurements by adjusting the base station local clock.
One way of identifying frames in the downlink (from the upper node to the base station) and the uplink (from the base station to the upper node) for a mobile connection is to attach a sequence number to each frame. These sequence numbers can, in the downlink, be correlated in the base stations against a base station reference timing/numbering order to align the frames so as to correct for intended transmission time (at the air interface). In the uplink, sequence numbers (related to the base station reference timing/numbering) are attached to frames from each base station before transfer of the frames to the upper node. At the upper node, frame combining/selection is performed based on these sequence numbers.
In some systems, the upper node (e.g., RNC) has a master system frame counter which is locked to an external reference or to a clock source. Some direct sequence CDMA systems (DS-CDMA) need a procedure capable of synchronizing base stations to have a frame level uncertainty of approximately plus or minus one millisecond relative to the upper node. In other words, the master frame counter of the upper node must be distributed to all base stations within a predetermined maximum time, e.g., approximately one millisecond.
For some mobile systems this degree of level uncertainty is necessary since the air interface (between the base station(s) and the mobile station) contains frames (of duration, for example, of ten milliseconds) with no frame number information. The mobile stations themselves cannot distinguish between such frames. Yet the mobile station must nevertheless know in what direction frames from the base station should be adjusted within plus or minus half the duration of the frames (e.g., five milliseconds).
In other systems in which the frame number is present on one of the channels, it is possible to have a phase difference of greater than half a frame. Such can occur in systems where the mobile station assists in measuring phase difference, in which case the channel upon which the phase difference measurement is performed may carry the frame number. This potential of greater than half a frame phase difference requires synchronization of base stations to a maximum phase difference with respect to an upper node (e.g., RNC).
What is needed therefore, and an object of the present invention, is an accurate and reliable technique for synchronizing timing units, such as timing units at base stations with an upper node.
Synchronization is effected in a cellular telecommunications network between a master timing unit located at control node of the network and a slave timing unit. The slave timing unit can be located either at the control node or a controlled node of the network. In accomplishing the synchronization, an initiating one of the master timing unit and the slave timing unit transmits a synchronization analysis command message including a first parameter to a responding one of the master timing unit and the slave timing unit. In response, the responding timing unit sends a synchronization analysis response message which includes at least second parameter and preferably a third parameter to the initiating timing unit. The initiating timing unit uses e.g., parameters extracted from the synchronization analysis response message to determine a synchronization adjustment value for the slave timing unit. In an embodiment in which the initiating timing unit is the master timing unit, the master timing unit transmits the synchronization adjustment value in a synchronization adjustment command message to the slave timing unit. In an embodiment in which the initiating timing is the slave timing unit, the slave timing unit calculates and performs the adjustment by itself, and then notifies the master unit. The synchronization adjustment value is preferably a synchronization offset value.
The first parameter which is included in the synchronization analysis command message is preferably a first time stamp value t1 related to the time that the synchronization analysis command message is transmitted from the initiating timing unit to the responding timing unit. The second parameter, inserted in the synchronization analysis response message by the responding timing unit, is a second time stamp value t2 related to the time that the synchronization analysis command message is received at the responding timing unit. The third time stamp value t3, also inserted in synchronization analysis response message by the responding timing unit, is related to the time that the synchronization analysis response message is sent from the responding timing unit. The initiating unit determines a fourth time stamp value t4 indicative of a time of reception of the synchronization sequence response message at the initiating timing unit.
When the initiating unit is the master unit, the initiating timing unit determines the synchronization adjustment value by comparing the second parameter t2 included in the synchronization analysis response message with a predicted second parameter t2-predicted. The predicted second time stamp value t2-predicted is determined as t2-predicted=((t1+t4)/2)xe2x88x92((t3xe2x88x92t2)/2). The synchronization adjustment value is then determined as t2-predictedxe2x88x92t2.
When the initiating unit is the slave timing unit, the initiating timing unit determines the synchronization adjustment value by comparing the first parameter t1 included in the synchronization analysis response message with a predicted first parameter t1-predicted. The predicted first time stamp value t1-predicted is determined as t1-predicted=((t2+t3)/2)xe2x88x92((t4xe2x88x92t1)/2). The synchronization adjustment value is then determined as t1-predictedxe2x88x92t1.
The parameters t1 through t4 are preferably values of system frame counters. In particular, the parameters t1 and t4 are then-current values of a system frame counter of the initiating timing unit. The parameters t2 and t3 are then-current values of a system frame counter of the responding timing unit. The synchronization adjustment value is used to adjust the value of the system frame counter of the responding timing unit.
The master timing unit can be located in a control node such as e.g., a Radio Network Controller (RNC) [alias, Base Station Controller (BSC)] or even in a mobile switching center (MSC). The slave timing unit can be located in a base station node, or in the control node (such as in a diversity handover unit situated at the control node).
In one embodiment, the synchronization messages transmitted between the master timing unit and the slave timing unit are encapsulated in asynchronous transfer mode (ATM) cells in a code division multiple access (CDMA) cellular telecommunications network.